Luminescence of Speleothems: A Comparison of Sources and Environments

Abstract

Calcite speleothems are typically coarsely crystalline, colored various shades from brown through orange and white, and strongly luminescent. For most speleothems, the color is due to higher molecular weight humic substances incorporated in the calcite crystal structure, while the luminescence is mainly due to lower molecular weight fulvic acids. The fine scale banding of luminescence intensity has importance as a climatic indicator. A suite of eighteen speleothems spanning a range of geologic and climatic settings and a range of colors from deep brown to nearly white were selected for detailed characterization and spectroscopic measurements. Spectra were measured on solid samples and on solutions prepared by dissolving the speleothems in dilute HCl. The luminescent emission appears as a single broad band with peak wavelength varying from 390 to 450 nm. The excitation spectra are typically more complicated, with several maxima, and show more locality-to-locality variation. The emission bands shift to longer wavelengths as the excitation bands move to longer wavelengths, indicating that a mixture of molecular species is being selectively excited. The spectra of the solutions are similar but not identical to the spectra of the crystalline solids. The decay time of the luminescence (phosphorescence) is in the range of 0.5 to 0.7 second. Comparison of speleothem spectra from caves in different climatic settings and of speleothem spectra from the same cave indicate that each speleothem produces spectra characteristic of specific overlying soils and pathways through the epikarst and the vadose zone. No features were discovered that characterize regional scale geologic or climatic settings.